Known automatic transmissions for automotive vehicles include step ratio controls for effecting speed ratio changes in response to changing driving conditions. The term “speed ratio”, for purposes of this description, is defined as transmission input shaft speed divided by transmission output shaft speed.
A so-called speed ratio upshift occurs when the driving conditions require a ratio change from a lower ratio (high speed ratio) to a higher ratio (low speed ratio) in the transmission gearing. The gearing may include, for example, either a planetary type gear system or a lay shaft type gear system. An automatic gear ratio shift is achieved by friction torque establishing devices, such as multiple disk clutches and multiple disk brakes. The friction torque establishing devices include friction elements, such as multiple plate clutches and band brakes, which may be actuated hydraulically or mechanically.
A step-ratio automatic transmission uses multiple friction elements for automatic gear ratio shifting. A ratio change from a low gear ratio to a high gear ratio occurs in a synchronous clutch-to-clutch upshift as one friction element is engaged and a second friction element is disengaged. One friction element may be referred to as an off-going clutch (OGC). It is released while a second friction element, which may be referred to as an oncoming clutch (OCC), engages to create the upshift. The upshift event is divided into a preparatory phase, a torque phase and an inertia phase.
During the preparatory phase, a transmission controller reduces the OGC torque capacity to prepare for its release and simultaneously, adjusts the position of an OCC actuator to prepare for its engagement. During the torque phase, the controller increases the OCC torque capacity in a controlled manner while the OGC is still engaged or allowed to slip at a controlled slip rate. This causes torque that is transmitted through the OGC to drop significantly in accordance with an increase in torque capacity of the OCC. The controller may maintain enough OGC torque capacity to keep the OGC securely engaged or locked during the torque phase, which immediately follows the preparatory phase. Alternatively, the controller may allow the OGC to slip at a controlled rate.
During the torque phase of a conventional control system, torque transmitted through the OGC decreases when the transmission output shaft torque drops. This creates a so-called torque hole. A large torque hole can be perceived by the vehicle occupants as an unpleasant shift shock. The inertia phase begins when the OGC is released or has no significant torque capacity.